86 get subset kwargs do not work if column name has spaces

tests/test_dataset.py

@@ -545,6 +545,19 @@ def create_index(self):
         with pytest.raises(RuntimeError):
             _ = TestDataset().index
 
+    @pytest.mark.parametrize(
+        ("column_names"),
+        (["else", "b", "c"], ["a", "has space", "c"], ["a", "b", "has-special-char"], ["a", "b", "1number_first"]),
+    )
+    def test_warning_index_no_valid_attribute(self, column_names):
+        class TestDataset(Dataset):
+            def create_index(self):
+                # Return index with some invalid column names
+                return pd.DataFrame(np.zeros((10, 3)), columns=column_names)
+
+        with pytest.warns(RuntimeWarning):
+            _ = TestDataset().index
+
     @pytest.mark.parametrize(
         ("level", "what_to_expect"),
         [

tpcp/_dataset.py

@@ -1,5 +1,6 @@
 """Base class for all datasets."""
 import warnings
+from keyword import iskeyword
 from typing import Dict, Iterator, List, Optional, Sequence, Tuple, TypeVar, Union, cast, overload
 
 import numpy as np
@@ -64,6 +65,8 @@ def _create_check_index(self):
         While we can not catch all related issues (i.e. determinism across different machines), this should catch the
         most obvious ones.
 
+        Furthermore, we check if all columns of the index are valid Python attribute names and throw a warning if not.
+
         In case, creating the index twice is too expensive, users can overwrite this method.
         But better to catch errors early.
         """
@@ -83,6 +86,19 @@ def _create_check_index(self):
                 "explicitly using `sort_values`."
             )
 
+        invalid_elements = [s for s in index_1.columns if not s.isidentifier() or iskeyword(s)]
+        if invalid_elements:
+            warnings.warn(
+                f"Some of your index columns are not valid Python attribute names: {invalid_elements}. "
+                "This will cause issues when using further methods such as `get_subset`, `group_label`, "
+                "`group_labels`, and `datapoint_label`.\n"
+                "To be a valid Python attribute a string should only consist of alphanumeric letters and underscores. "
+                "Strings starting with a number, containing spaces or special characters are not allowed."
+                "Furthermore, they must not shadow a built-in Python keyword.",
+                RuntimeWarning,
+                stacklevel=1,
+            )
+
         return index_1
 
     @property